If you took the trim off, wouldn't the heat make it all curl and distort?
Think he does it on the body to keep it straight.
Should be ok if you keep it moving to avoid hotspots.
....Caveat emptor as they say........

I am not sure it made all that much difference. A before and after shot from the same angle might be more convincing.
I don't know that taking the mouldings off the car would cause them to warp as some are made of 'memory' plastic. I have seen some returned to shape by warming after a collision when they have been distorted. I don't ever recall them looking any 'shinier/cleaner' though.

It works by melting the plastic(just a little). Silicon from inside the plastic boils to the surface. It's a 1-2 year thing, if not maintained with "black in a flash" or similar bumper care stuff. Did that to my old trusty Renault once. Before and after pics added. Pictures have a few weeks between them

Before:
After:

Anyhuu, because it is "melting" plastic, then don't stay on one spot too long

I've done the heat gun trick on my 406, and it definitely works for a while, until the plastic sustains enough UV damage to fade again.

It's actually quite easy to do, as you can see it working and turning the plastic black again. With good lighting, progress is very obvious. You just have to watch out that it doesn't melt the plastic (so use the lowest heat gun setting that works). On D9s you have to be very careful around the chrome strips, never aim the gun directly at them, as the surface will start melting and bubbling very easily.